(1) Field of the Invention
The present invention relates to steam temperature control systems in general and in particular to such systems which control tuned parameters which change in response to system load.
(2) Description of the Prior Art
Steam temperature control on a drum type boiler is difficult due to time lags and delays built into the design of the process. There are time delays between the attemperator spray location and its effect on final steam temperature leaving the secondary superheater. Time lags are also caused by the head transfer characteristics of the superheater metal and the steam itself.
Any control with relatively long time constants (two minutes or longer) will operate in a more stable fashion if open loop predictive (feedforward) methods are employed to preset the controlled medium. In addition, if intermediate control points are useful and somewhat predictive of the final steam temperature, then these are also useful in a cascade method of control.
Almost all drum type boilers are designed to have a generally rising uncontrolled secondary superheater outlet temperature profile with increasing boiler load. The design usually is such that the unit does not have to reach the required main steam outlet temperature at loads below 50 percent boiler load, and therefore is not controlled at these loads. Above such a load, the excess superheat temperature is "sprayed out" by the spray attemperator.
Classical control techniques commonly used in steam temperature controls are feedforward, feedback using proportional plus integral plus derivative controller, cascade, and anti-integral windup.
Because of the time delay and time lag, a standard proportional plus integral controller will either be detuned, providing a slow, sluggish control, or be unstable.
As the response time characteristics will vary with load, the control adjustments are usually set as a compromise between high and low load settings.
To prevent the controller from integrating when the spray valve is closed at low loads, controller limits are developed to prevent the P.I.D. controller from integrating upward.
Thus the classical control system does not address two vital problems; i.e. true time delay and control tuning parameters which change with load.